1. Field of the Invention
The present invention relates to radar absorber structures, and more particularly to multi-layer radar absorbing structures made of environmentally compliant materials.
2. Description of the Related Art
The use of multi-layer radar absorbing material substrates is well-known.
For example, U.S. Pat. No. 5,922,986 issued Jul. 13, 1999 to Wanninger et al. discloses an armor plating for vehicles that has, at its outer side, a layer of plastics material bound explosive containing radar absorbing materials. The armor plate comprises a plate shaped module having an exterior surface arranged to face outwardly from the vehicle and an interior surface arranged to face toward the vehicle, the plate shaped module including an explosive material bound with a plastics material binder containing a radar absorbing material.
U.S. Pat. No. 5,192,810 issued Mar. 9, 1993 to Hill discloses a method and apparatus for manufacturing dimpled structures from elastic cloth. The method permits forming complex polyimide foam shapes by impregnating a low density, open cell, reticulated foam with polyimide foam precursor, placing the combination in a mold, closing the mold, then heating the assembly to the foaming and curing temperatures of the precursor. As the powdered precursor expands, it spreads throughout the reticulated foam producing a product having substantially uniform density and polyimide foam characteristics. If desired, the reticulated foam can be removed from the product by using a material that outgasses and boils away at polyimide processing temperatures. Radar absorbing materials may be incorporated in the reticulated foam so that the final product will have uniformly distributed radar absorbing components with desirable high temperature resistance and other properties of polyimide foams.
U.S. Pat. No. 5,845,877 to Justice et al., issued on Dec. 8, 1998, discloses a sealing assembly for reducing the gap between an aircraft's movable flight control member and adjacent structures. The flight control member and adjacent structures have external surfaces and side surfaces facing each other. First and second flexible bulb seals are mounted on the periphery of the side surface of both the flight control member and the adjacent structure forming an extension of the external surface of both. Each bulb seal includes a compressible foam bulk absorber core. A first sheet of dielectric material is bonded to the bottom surface of the core. A second sheet of flexible resistive material is bonded to the top, and sides of the core. A third sheet of flexible magnetic radar absorbing material is bonded to the second sheet. A fourth sheet of flexible abrasion resistant material is bonded over the third sheet.
All of these absorbers, however, suffer from a variety of drawbacks. First, repeated flexure of such absorbers tends to result in failure of the elasticity or structural integrity of the various substrates. Second, the multi-layer absorbers are rendered defective due to improper distribution of the conductive material in or on the foam substrates. Third, the weight of the absorber and conductive filler often exceeds the limits of the anticipated application. Fourth, repeatability in manufacture, with a significant level of quality, is rarely achieved.
Against this background of known technology, the applicant has developed a novel method for forming a multi-layer elastomeric absorber including radar absorbing material in at least one of the layers so that the multi-layer absorber will achieve normal incidence absorption at selected frequencies, while overcoming many of the deficiencies and drawbacks of similar absorbers currently known in the relevant technology.